The known machines which are referred to as blenders are machines having two twin heads, each of which blends together eight to 14 slivers of fibres emanating either from a dyeing stage, or machines for breaking or converting continuous cables into discontinuous fibre slivers. Sometimes, both heads work by providing a bar movement during drawing, between supply rollers and what are known as drawing rollers, in order to control progressive retention of the fibres during drawing. In another application, it has been noted that the drawing operation, the main object of which is to break open or blend the greatest possible number of components, takes place most easily although imperfectly, between trains of three rollers, the number, speed and spacing of which have been judiciously established according to the fibre length diagram. As stated earlier, two heads are used in a twin disposition in order to increase to the maximum the number of "doublings", in order to combine them into a single sliver in a second drawing stage provided with a sleeve or cylinder control system referred to as a reducer or grader, which will receive the two juxtaposed slivers upon their separate emergence from each head.
Known single-head machines, composed of groups of three rollers an upper pressure roller and thereafter two twin carrier rollers for maintenance and control, respectively, are adapted to reconvert excessively long fibres obtained from the "converter", while others reconvert or convert discontinuous slivers of fibres, to make them of a length referred to as a "cotton length", that is to say up to 40 m minimum length, nowadays mainly adapted for Open End spinning.
In the known blenders referred to above, it is difficult to set the two twin heads identically.
The second connected drawing stage combining the two slivers obtained from the head described above acts on a mass of fibres which is greatly reduced in weight by prior drawing processes, so that by necessarily rendering it narrower, it will ocupy only one-third the width of a conventional head.
The difficulties arising during passage of a mass of material to be treated through a single head reconverting machine are well known: it is impossible adequately to narrow the lap even with condensers set increasingly closer to one another between the four, five or six successive trains, so that the emerging sliver is so thin that it becomes wound around the delivery rollers, preventing high speed delivery.